Electronic equipment used in commercial and military applications must adhere to conducted emission requirements at the input power terminals of the electronic equipment. These requirements are levied on the electronic equipment manufacturers by agencies such as the FCC, European Union, and, in the case of the U.S. Military, the procuring branch of the service. The goal of these requirements is to minimize the negative interactions that may occur between various pieces of electronic equipment. The negative interactions occur because the noise currents, normally produced by switching action in one piece of electronic equipment, interfere with the proper operation of another piece of electronic equipment. Typically, the method of coupling the negative interactions is conduction through a common shared power bus (DC or AC). Additionally, noise current feeding on the power bus may set up electromagnetic interference that couples into the surrounding electronic equipment via electromagnetic radiation. Noise currents are typically AC and have frequencies that are much higher than the operating frequency of the power source.
In order to meet the conducted emission requirements, inductive components are used in conjunction with capacitors in electrical networks referred to as EMI filters. EMI filters are highly effective in attenuating noise currents that emanate from electronic equipment. Noise currents can be characterized by the path that they take during conduction. Two conduction paths exist: normal mode (sometimes referred to as differential mode) and common mode. See FIGS. 1 and 2 respectively. Normal mode noise currents typically feed into one input power terminal of the electronic equipment and exit from the electronic equipment via the remaining input power terminal (as closed). The sum of all the normal mode noise currents entering and exiting a particular piece of electronic equipment input power terminals is zero. In contrast, the sum of common mode noise currents entering and leaving the electronic equipment input power terminals is not zero. These currents typically find alternate paths through the equipment chassis and the application earth ground structure. These alternate and less predictable paths can be highly disruptive.
In addition to the noise currents that feed into and out of equipment power terminals, power producing currents must also feed. The high frequency noise currents, both normal mode and common mode, essentially modulate the lower frequency power producing component. Power producing currents also feed in the normal mode. Capacitors, used in EMI filters, must efficiently pass the lower frequency power producing component and attenuate the high frequency noise currents.
Common mode noise currents typically occur when energy from the switching transition within the electronic equipment capacitively couples into the equipment ground structure or system ground structure. The magnitude of the common mode noise current is proportional to the parasitic capacitor magnitude and the time/rate of change of the voltage across the parasitic capacitance. Accordingly, a large voltage swing, and a fast switching transition results in a large common mode current magnitude. The duration of the common mode current pulse is proportional to the duration of the electronic equipment switching transitions. The frequency spectrum for the common mode noise currents is often discrete since the switching transitions are usually periodic. The fundamental component of the frequency spectrum is the same as the switching frequency of the electronic equipment and the harmonics are at multiples of the fundamental frequency. The magnitude of the common mode noise frequency spectrum envelope drops gently, starting at the fundamental frequency, to its first null, which in an ideal system is equivalent to the inverse of the switching transition time. This frequency spectrum, with its large energy content at the higher frequencies, can be quite disruptive if mitigation techniques, such as EMI filters, are not employed.
The nature of the common mode frequency spectrum places performance constraints on an FTC that is used as an EMI filter. The FTC must provide a low impedance path to chassis ground for common mode noise current. The low impedance path must be across the entire common mode noise frequency spectrum. The impedance exhibited by the FTC to chassis ground must be lesser than the common mode noise source impedance and the impedance of the load circuit to chassis ground. These performance constraints translate into requirements for the electrical elements of the FTC including capacitance, equivalent series inductance (ESL) and equivalent series resistance (ESR). The FTC capacitance value must be large enough to shunt the low frequency fundamental common mode spectral component. Additionally, the FTC must exhibit a low ESL, enabling it to effectively shunt the higher frequency common mode spectral components. Finally, the ESR of the FTC must be low, enabling it to effectively shunt all common mode spectral components.
FTC's used in high current applications must pass the power source current without introducing power loses. Additionally, FTC's need to provide a low impedance thermal path to the surrounding environment (air or chassis), for power losses that do occur within the FTC. Failure to minimize power losses or provide a low impedance thermal path will result in excessive FTC operating temperature which will reduce the FTC reliability. Excessive power losses within the FTC can also impact the operation of the application circuit by reducing the available circuit input voltage or by contributing to a temperature rise in the application circuitry. These requirements drive the FTC design to maximize cross-sectional conductor area and minimize the length of the FTC conducting element.
FTC's used in high voltage applications must also support the high voltage without breaking down. This requirement places constraints on the insulating materials used in the construction of the capacitor. Additionally, when insulating material cannot be employed, the FTC design must rely on clearance and creepage distances to avoid voltage breakdown. Clearance is the minimum distance required to avoid breakdown (through air) for a given potential. Creepage is the minimum distance required to avoid breakdown (over an insulating surface) for a given potential and insulating material. Clearance and creepage distances become important design requirements at the FTC terminations and dictate the physical size of the capacitor design.
When these low impedance, high current requirements and high voltage requirements are impressed on prior art FTC's, a costly, bulky design results. In addition, the prior art FTC becomes difficult to produce which results in excessive lead time and integration risk. There is a need then in the industry for cost effective, low lead time FTC's exhibiting low impedance, high current capability and high voltage capability.
Commercial off-the-shelf FTC's available today are targeted for low current and low voltage applications. FTC's capable of exhibiting low impedances over a wide frequency range as would be the case with 0.3 microfarad ceramic capacitor, passing a source current of 200 amps, and of supporting voltages of 1000 volts are not readily available. Even custom FTC's, as distinct from commercial off-the-shelf FTC's, do not support the capacitance requirement or the 200 amp current unless three or more custom FTC's are configured in parallel. Further, current custom FTC's have a clearance and creepage distance of 3.2 mm. For the application noted above, clearance and creepage distances of 5 to 10 mm, depending on the pollution degree environment, are required. Further, present custom FTC's are both expensive and have long lead times for delivery, typically in the realm of 22 weeks or more.